1. Field of the Invention
This invention relates to data transmission systems and, more particularly, to data transmission systems including a repeater connecting transmission lines to which a plurality of node stations are connected in parallel.
2. Description of the Prior Art
In data transmission where a plurality of node stations are connected in parallel to the transmission line (a multidrop connection), when node stations transmit data at random, a plurality of signals can simultaneously appear on the same transmission line. Thus, proper transmission often cannot be performed. The interference of signals from at least two signal sources on the same transmission line is called a contention. In order to prevent the contention and to smoothly exchange data between the node stations, a set of given rules is introduced for a data link to control the transmission. The set of given rules is called a protocol. Various types of protocols are used in data transmission or communication.
However, when a lot of stations are connected to the transmission line or the transmission line is long, signal degradation occurs. Therefore, repeaters are inserted in the transmission line in order to amplify and reshape corresponding signals. FIG. 1 shows a model of a principle of a multidrop connection system. In FIG. 1, stations 1a through 1n are connected in parallel to line 3 and stations 2a through 2m are connected in parallel to line 4. Lines 3 and 4 are connected to opposite sides of repeater 5. Repeater 5 includes two channels, one for regenerating signals in each direction.
Typically, in the past, if station 1a is to communicate with station 2b, station 1a will issue a command. The command will pass through repeater 5 to station 2b. Station 2b will then process the command, formulate a response and second the response through repeater 5.
Repeater 5 (shown in FIG. 2), such as is disclosed in Japanese Patent Disclosure No 57-171864, includes receiver 6a through driver 7a for regenerating signals from line 3 to line 4. Receiver 6b and driver 7b regenerate signals from line 4 to line 3. When the command signal passes from station 1a and line 3 to line 4 and station 2b, switching circuit 8a disables receiver 6b and driver 7b to prevent feedback. The disabling continues until switching circuit 8a has determined that no signals are flowing from line 3 to line 4. This takes a certain period of time after the signal from line 3 actually stops.
During this time, station 2b is receiving the command and processing it. By the time station 2b is ready to transmit the response, receiver 6b and driver 7b should no longer be disabled so that the response from station 2b can pass through repeater 5.
However, recently, stations have been developed which can respond a very short time after receipt of a command. A problem exists, however, since typical repeaters must wait for a predetermined time before signals can pass in the opposite direction. Therefore, the being of the response may be clipped by the repeater.
The conventional system does have an advantage in that a relatively simple structure can be used for the data link. However, as discussed above, with fast responding stations, repeater 5 may clip a part of the response. This results because a station is able to detect the end of a signal (such as illustrated in FIG. 3) by an end flag associated with the signal and respond immediately, whereas the repeater must wait a predetermined period after the end of the signal to ensure that the signal has, in fact, ended. Thus, as shown in FIG. 5, repeater 5 waits for a checking period corresponding to 7 or 8 bits to guarantee that the signal has ended.
If the checking period is set up with less than 8 bits, then a signal with more than 5 consecutive bits in the same state may cause switches 8a and 8b to enable the other channel, thus interrupting the signal.
Thus, it is difficult for a conventional system to transmit data more efficiently, because conventional repeaters detect the end of the data signal by detecting the occurrence of a predetermined number of bits in the same state in the data signal (shown in FIG. 5).